Generally, large-size transportation vehicles, called dump trucks, have a liftable vessel (a loading platform) on a frame of a vehicle body, and carry heavy objects to be transported like crushed stones in a state in which the objects to be transported are loaded in a large quantity on the vessel.
In this regard, for example, a traveling drive unit which is adapted to drive a drive wheel of a dump truck is provided with a tubular axle housing which is mounted on a vehicle body, a drive source such as an electric motor or a hydraulic motor which is provided in the axle housing and rotationally drives a rotational shaft, wheel mounting case rotatably mounted around an outer periphery on the fore end side of the axle housing through a bearing to support a drive wheel thereon, and a multi-stage reduction gear mechanism incorporated into the wheel mounting case to transmit rotation of the rotational shaft to the wheel mounting case at a reduced speed (e.g., see Japanese Patent Laid-Open No. S62-221918 A and 2006-264394 A).
By a multi-stage reduction gear mechanism of this sort, rotational output of a drive source like electric motor, for example, is reduced in speed and transmitted to a tubular wheel mounting case (wheel) to generate a large torque in a drive wheel of a dump truck, i.e., in a front or rear wheel. In addition, a traveling drive unit for a dump truck is normally adapted to rotationally drive left and right wheels by respective reduction gear mechanisms independently of each other.
In order to keep each gear member of the reduction gear mechanism in a lubricated state, lubricant oil is pooled in a tubular wheel mounting case on which a drive wheel is to be mounted. Lubricant oil is forcibly circulated into and out of the wheel mounting case (axle housing) by the use of a lubricant circulation pump or the like, and an oil cooler is provided in the course of a circulation path to cool down the lubricant oil.
In this case, for example, a lubricant circulation pump is located on the outer side of an axle housing. An intake side of the lubricant circulation pump is connected to an intake pipe, with the other side (inlet opening side) of the intake pipe being extended into and immersed in lubricant oil pooled in the wheel mounting case. On the other hand, an output side of the lubricant circulation pump is connected to an output pipe or a supply pipe, and the other side of the supply pipe is disposed so as to be extended into the wheel mounting case at a position above the intake pipe.
In this regard, in the case of the prior art mentioned above, when a wheel is put in high speed rotation, namely, when the vehicle speed is increased, the lubricant oil which is pooled in the wheel mounting case behaves to stick on the entire inner peripheral surfaces of the wheel mounting case under the influence of centrifugal force resulting from the rotation of the wheel mounting case. For this reason, the surface level of the lubricant oil in the wheel mounting case drops below the inlet opening of the intake pipe.
Therefore, when a vehicle speed of a working vehicle like dump trucks is increased, an inlet opening of an intake pipe in a wheel mounting case may be relatively dislocated to a position above the surface level of lubricant oil, disabling the lubricant circulation pump to suck in lubricant oil. Besides, in such a case, the lubricant circulation pump is put in a dry idling operation, which would cause premature abrasive wear and damages to seals and bearings of the lubricant circulation pump more and as a result would shorten the service life of the pump itself.